Technical Evalution of Sru

 

Technical & Commercial Evaluation Of Processes For Claus Tail Gas Treatment

 André Le Gall, Gall, Dominiq Dominique ue Gadelle Process and Technology Division Technip France Paris La Défense, France

ABSTRACT  After its removal from crude oil, refinery refinery sulphur is principa principally lly in the form of hy hydrogen drogen sulphide sulphide (H2S). The most common means of recovering the sulphur contained in H2S is the modified Claus process which can recover from 90 to 97% of the sulphur contained in the acid gas feed. Recovery depends upon such things as acid gas composition, age of the catalyst and number of reactor stages. The gas leaving a modified Claus plant is referred to as tail gas that in the past was burned to convert the unreacted H2S to less toxic but still undesireable SO2. The off-gas stream was then vented to the atmosphere. Currently, all Western countries and many developing countries are tightening environmental regulations to limit further the amount of sulphur that can be emitted to the atmosphere. There is therefore a need to add Tail Gas Clean-Up Units in refineries. TGCU’s are quite expensive, often representing an investment of the same order of magnitude as the original Claus plant. The proper selection of TGCU process is therefore of some importance to operators whereas the choice is made complicated by the different performance levels and life cycle costs. This paper compares a selection of the most popular Tail Gas Clean-Up (TGCU) processes based on their sulphur recovery yield (%) and most importantly in terms of the recovery cost per tonne of SO 2 ($/t). Technical evaluations, capital and operating costs are compared. The paper is based on work performed by TECHNIP FRANCE for the French energy conservation agency ADEME.

Presented at GPA Europe Technical Meeting Paris 21st February, 2003   GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes

Page 1 of 22

 

1. INTRODUCTION This paper describes a study conducted to compare the relative technical and economic merits of a wide range of Tail Gas Clean-Up (TGCU) processes when compared to a conventional Claus unit. The study covered the following areas: technical evaluations and capital and operating costs. Selected processes were compared on the following basis: sulphur recovery (%) and estimated recovery cost for SO2 ($/t). The study was performed by TECHNIP FRANCE for the French energy conservation agency ADEME.

1.1

SULPH SULPHUR UR IN REFIN REFINERIE ERIES S

The sulphur species after its removal from crude oil is generally in the form of Hydrogen Sulphide (H 2S). The most common means of recovering the sulphur contained in H2S is the modified Claus process. The modified Claus plant can recover from 90 to 97% of the sulphur contained in its feed. The recovery depends upon such things as feed composition, age of the catalyst and number of reactor stages. The gas leaving a modified Claus plant is referred to as tail gas and, in the past was burned to convert the unreacted H2S, which is lethal even at low levels (lethal concentration: 1 000 ppm after one min.), to SO 2  which has a much higher toxic limit (lethal concentration: 2 500 ppm after one min.). The off-gas stream was then vented to the atmosphere. Currently, all Western countries and many developing countries limit the amount of sulphur that can be emitted to the atmosphere. In Europe, the 1998 directive limits SO2 to a 1700 mg/Nm3 emission "bubble" in which Cla Claus us effluent is a significant part. Allowable emission levels will be reduced during the coming decade. New units must be equipped with TCCU’s. There is also regulatory pressure to add a TGCU to existing units. TGCU’s are quite expensive, often costing an order of magnitude of Claus plant.

1.2 H2S IN REFINERIES The feedstock to a refinery Claus unit is usually a combination of the following streams: § §

Regenerator overhead stream from a gas sweetening unit (off-gas coming from a FCC and a gasoil HDS units), Effluent stream from the Refinery Sour Water Stripping unit.

Typical feed conditions and properties for each stream are given in the following table. Temperature Pressure Flowrate

(°C) (kpa) (kmole/h) (kg/h)

Stream 1 40.0 170.0 145.63 4947.91

Molecular Weight

(kg/kmole)

33.98

Stream 2 90.0 170.0 5.65 121. 67

Overal Overalll 42.0 170.0 151.28 5069.58

21.53

33.51

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes

Page 2 of 22

 

1.3 THE CLAUS PROCESS The study considered the enhancement of sulphur recovery from an existing two reactor refinery Claus unit as shown in the typical flowscheme below. The tail gas unit receives the effluent from a Claus unit producing 100 t/d of liquid sulphur with sulphur recovery of 96% weight. This upstream Claus unit will include the following main equipment: -

One thermal stage, Two catalytic stages,

-

One thermal incinerator, One daily sulphur storage pit.

Steam

#1 Reheater  Reaction Furnace and Waste Heat Boiler 

#1 Reactor 

Acid Gas & SWS effluent

#1 Condenser 

#2 Condenser  Steam

 Air Blower 

Sulphur 

Sulphur 

#2 Reheater  Tail Gas to Incinerator or TGCU

#2 Reactor 

#3 Condenser 

Sulphur 

Typical Two Stages Claus Unit

 

The above flowscheme is very typical. Various arrangement are available, depending on the relative ratio of acid gas to sour water stripper gas, depending on the capacity, on the reheating arrangement (steam heater, in-line in-line burners, electric heaters, …), etc. In refinery applications (relatively rich acid gas) sulphur recoveries range anywhere from 92 to 96% in a two converter arrangement. A third converter will increase the recovery by 1 to 2 additional percent. The recovery is based on the Claus reaction ; one third of the H2S in the feed gases is oxidized thermally with air to form SO2. The latter then reacts with the remaining H2S to form elemental sulphur (Claus reaction). Excess air is to be injected to ensure the destruction of hydrocarbons and/or ammonia present in the feeds. H2S + 3/2 O2 

à SO2 +

2 H2O

2 H2S + SO2  à 3 S + 2 H2O

(1) (2)

In order to maximize sulphur recovery, precise air to acid gas ratio control is required in order to achieve a 2 to 1 H2S to SO2 ratio.

1.4 CLAUS UNIT EFFLUENT STREAM The Claus unit effluent stream (tail gas) properties are: Temperature Pressure Flowrate Molecular Weight

(°C) (kpa) (kmole/h) (kg/h) (kg/kmole)

128 145 434 10657 24.6

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes

Page 3 of 22

 

2. TAIL GAS CLEAN-UP PROCE PROCESS SS SCREENING SCREENING More than 20 processes for Tail Gas Clean Up have been developed over the years to meet the environmental restrictions on sulphur emission from stationery sources. TGCU processes can be broadly divided into four groups of processes, namely: § § § §

Dry bed processes, Liquid Phase Sub-DewPoint Processes, Liquid scrubbing processes, Liquid Redox process.

The first and third categories can further be divided in sub-categories depending on the sulphur recovery method used. It should be noted that some arrangements combine the capabilities of both Dry Bed and Liquid Scrubbing processes.

2.1 DRY BED PROCESSES The main process step is achieved on a solid catalyst. Two paths have been followed: § §

Extend Claus reaction on a solid bed Oxidise Sulphur compounds to SO2 prior to absorption, or reaction.

Extension of Claus Reaction on a Solid Bed Several processes have been developed and have been widely used. Basically, they are variations of the same process and differ mainly in the regeneration technique used: § § §

Elf/Lurgi Sulfreen Process and derivatives (HydroSulfreen, DoxoSulfreen, …),  AMOCO Cold Cold Bed Absorption (CBA), (CBA), MCRC Process

Direct Oxidation Processes In order to overcome the limitation in conversion of the Claus catalysts, new catalysts have been developed in order to promote either: § §

Direct oxidation of H2S to sulphur (such as Superclaus, MODOP, BSR Hi-Activity technologies), Claus reaction and direct oxidation of H2S to sulphur (such as Selectox technology).

In a conventional Claus process unit, the conversion to sulphur is based on the oxidation (complete oxidation) to SO2 and the subsequent production of elemental sulphur by the Claus reaction (reactions (1) and (2). Direct oxidation of H2S to sulphur can also be thermodynamically completed according to: H2S + ½ O 2 à S +H + H2O

(3)

In the absence of a catalyst, the rate of this reaction is very slow, and the reaction becomes noticeable only above 300°C. However, at these temperatures, the formation of SO 2  is also accelerated and proceeds according to the reaction (1) above, as well as from: S + O2  à SO2 

(4)

3 S + 2 H2O ßà 2 H2S + SO2  (2’) In order to limit SO2 formation and favour partial oxidation reaction (3) temperature should be limited well below 300°C, and a suitable catalyst developed. Several catalysts have been developed and form the first family of processes listed above.   GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes

Page 4 of 22

 

In certain specific instances (lean gases with low H2S content where flame stability and impurities destruction is of concern), catalysts able to promote both classical Claus reaction and direct oxidation have been developed.

2.2 LIQUID PHASE SUB-DE SUB-DEWPOINT WPOINT PROCESS Clauspol family of processes The Clauspol family of processes was developed by the French Institute of Petroleum (Institut Français du Pétrole – IFP). The fundamentals of the process consist in extending the Claus reaction under subdewpoint conditions in liquid phase. The liquid phase consists in a non-volatile solvent containing dissolved catalyst (sodium salt of an inorganic acid) which is a solvent for H2S and SO2, but not for liquid sulphur. The Claus reaction: 2 H2S + SO2  ßà 3 S + 2 H2O can therefore proceed at low temperature (120°C) and is shifted further to the right as the produced sulphur is removed from the reaction medium, as it is not soluble and separates.

2.3 LIQUID SCRUB SCRUBBING BING PROCESS PROCESSES ES Broadly speaking, there are two main categories of Liquid Scrubbing processes, H2S scrubbing processes and SO2 scrubbing processes. In the most commonly applied configurations, H2S or SO2 are recycled to the upstream Claus Unit. H 2S Scrubbing Processes This type of process is by far the most widely applied TGCU type of process. This type of process can achieve overall sulphur recovery above 99.9%. The concept underlying H2S scrubbing processes are: Hydrogenation and hydrolysis of all sulphur compounds to H2S §  Absorption of H2S by an Amine solution (generic amine or specialty amine) § Regeneration of the Amine solution and recycle of the H2S to the upfront Claus Reaction Furnace. SO2   Scrubbing Processes §

This type of process is consists in converting, or recovering and recycling SO2. This part of the TGCU unit is located downstream the thermal incinerator of the Claus Plant. This means that all sulphur species (H2S, COS, CS 2, Sulphur) have been previously oxidized to SO 2, therefore no hydrolysis/hydrogenation hydrolysis/hydro genation step is required. This family of processes offers therefore a potential for very high sulphur recoveries, provided SO 2 can be converted and/or recycled. Several processes are derived from flue gas desulphurization techniques, and even if they have not been applied to Claus units tail gas, their main elements may have been referenced for other applications. Two types of solvents are used for SO2 absorption, either chemical absorption by an aqueous solution a basis compound, or physical absorption. Roughly speaking physical absorption implies a higher circulation flowrate of solvent, easier rich solvent regeneration, and potentially a reduced tendency to enter into side-reactions leading to undesirable by-products. The advantages of chemical solvents are: lower circulation rates and usually a better absorption capability. The Wellman-Lord process has been the most widely used process of that kind since the early 60’s. The absorption media is a solution of Sodium Sulphite Na 2SO3. Other processes have been developed more recently, such as the Elsorb process which uses an aqueous solution of sodium phosphates, or the Cansolv process which uses an amine based solvent. The ClausMaster process belongs to the same family, using a phosphorus based bas ed solvent.

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes

Page 5 of 22

 

2.4 LIQUID REDOX PROCESSES Since the beginning of this century, various Redox (or Liquid Phase Oxidation) processes have been developed to absorb both H2S and NH3 from mainly town gas or coal gas by the formation of Ammonium Sulphate and of elemental sulphur. The initial objective was to overcome the limitations of the existing Iron Sponge processes and to develop a regenerable, reusable oxidation medium. Among the processes developed, those still in commercial application are: §

The Stretford Process, which has been widely used since the 50’s and uses ADA (anthraquinone disulphuric acid) as an oxygen carrier in an aqueous solution containing carbonate and vanadate.

§

The Sulferox process and the LO-CAT process which both use Iron Complex solutions as oxygen carriers.

2.5 OTHER PROCESSES  A significant significant number number of other TGCU processes processes have been developed developed over the years, some of which have now been abandoned, or have not yet reached a commercial status, others are similar in essence to processes process es described above.

3. SELECTION OF REPRESENTATI REPRESENTATIVE VE PROCESSES TGCU processes, as outlined above, were grouped in “families” of processes which share several common features. Therefore, in the framework of the evaluation study, it was considered (at least for cost estimate purposes) to restrict the evaluation to one, or possibly two, significant representatives of eachacceptable family. The licensed processes selected for study were as follows: Dry Bed Sub-DewPoint Sub-DewPoint processes: processes : § §

Sulfreen with alternatives (HydroSulfreen and DoxoSulfreen) Cold Bed Adsorption

Liquid Phase Sub-DewPoint Sub-DewPoint processes: §

Clauspol II

Catalytic Oxidation: §

Superclaus 99

H 2S Scrubbing : §

Technip KTI-RAR

Liquid Redox : §

US Filter’s LO-CAT (now owned by Merichem)

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes

Page 6 of 22

 

3.1 SULFREEN PROCESS The Sulfreen Process is a dry-bed, sub-dewpoint absorption process based on the extension of the Claus reaction. The reactor operates below the dew point of sulphur (125-150°C) to form elemental sulphur. The reaction is extended, and the recovery enhanced, first because the equilibrium is thermodynamically favoured at low temperature, and second, because the Sulphur is absorbed on the catalyst, therefore shifting further the reaction to the right: 2 H2S + SO2  à 3 S + 2 H2O The Sulfreen process basically consists in two (occasionally three for large capacities) Sulfreen reactors in series with the Claus reactors. Activated Alumina is used as a catalyst.  As the sulphur accumulates on the catalyst, the activity decreases and the beds have to be regenerated thermally. During the regeneration step, sulphur is desorbed and the catalyst activity is restored to full activity by part of the TGCU tail gas which has been preheated in a dedicated heater. Once the regeneration regeneratio n is achieved, the catalyst bed is cooled to the operating temperature. Sulphur from the hot regeneration stream is condensed in a dedicated condenser. Typical overall Sulphur recoveries are in the order of 98.5 to 99.5% depending on the Claus Unit arrangement. COS and CS 2 are not converted, nor recovered. To date, there have been more than 40 Sulfreen units licensed, with capacities ranging from 5 to 2200 MTPD (expressed as Claus Unit feed sulphur). The following figure is a simplified process diagram of a two-reactor system (one in absorption, one in regeneration). To Incinerator 

Sulfreen Reactor #A

Sulfreen Reactor #B

Claus Unit Tail Gas

Circulation Blower  Sulphur 

SULFREEN Process Reactor A in adsorption

Closed Adsorption Regeneration

Opened

 

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes

Page 7 of 22

 

3.2 HYD HYDROSULFREEN ROSULFREEN PROCESS COS and CS 2  in the effluent from a Sulfreen Unit can represent about 50% of the residual Sulphur. To overcome this limitation, a conversion step is added upstream of the first Sulfreen reactor. This conversion step performs the hydrolysis of COS and CS 2  to H2S and is performed on an activated Titanium oxide Claus catalyst such as Procatalyse CRS31. This reactor operates at about 300°C. The Claus reaction takes place in the HydroSulfreen reactor and produced sulphur is condensed in a dedicated condenser. The preheat of the Claus unit tail gas to the HydroSulfreen reactor is performed by a separate coil in the Sulfreen regeneration heater.  After condensation, the effluent from the hydrolysis reactor is routed to the Sulfreen section, similarsulphur to the one described above. Typical overall Sulphur recoveries are in the range of 99.5 to 99.7%. To date, there have been 4 HydroSulfreen units licensed, with capacities up to 400 MTPD (expressed as Claus Unit feed sulphur). The schematic process diagram is given below: To Incinerator 

Air 

Regeneration Heater 

Hydrolysis Reactor 

Sulfreen Reactor #A

Sulfreen Reactor #B

Sulphur 

Claus Unit Tail Gas

Circulation Blower  Sulphur 

Closed Adsorption

HYDROSULFREEN Process

Regeneration

Opened

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes

Page 8 of 22

 

3.3 DOXOSULFREEN PROCESS This process is based on a direct oxydation of H2S into sulphur, with oxygen.  At the outlet of a conventi conventional onal Sulfreen, or HydroSulfreen HydroSulfreen process, the sulfur conversion conversion cannot proceed further due to the reduced H2S and SO2 concentration, and due to the water partial pressure which tends to shift the conversion to the left. The DoxoSulfreen concept is based on two ideas: §

§

the upstream units are operated to get a slight excess of H2S, compared to the quantity necessary to maintain the Claus ratio, therefore a nearly total SO 2 conversion takes place on the conventional Sulfreen catalyst: 10 H2S + SO2  à 3 S + 2 H2O + 8 H2S the remaining H2S is oxidised to elemental Sulphur according to

2 H2 S + O2  à 2 S + 2 H2O  A simplified simplified proces process s flow diagra diagram m is given below below::

Closed

To Incinerator 

Adsorption Regeneration

Opened

DoxoSulfreen Reactors  A & B

Sulfreen Reactors  A & B

Claus Unit Tail Gas

Circulation Blower  Regeneration Heater 

DOXOSULFREEN Process

Sulphur 

The gas from the Sulfreen reactors is further cooled to 100-130°C, and fed to a second catalytic stage together with a stream of air so that the direct oxydation of the residual H2S can take place. Both SULFREEN and oxydation beds are regenerated through a common regeneration loop.  Activated Carbon was initially used as a catalyst in the oxidation step (thus the initial name of CarboSulfreen), this catalyst being currently replaced by a new oxidation catalyst developed by Procatalyse. The DoxoSulfreen Process has demonstrated its ability to recover Sulfur up to the 99.8% level. This process can be used in combination with an Hydrolysis step (HydroSulfreen) in front of the Sulfreen reactors. The CarboSulfreen version is currently in operation in two German refineries.

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes

Page 9 of 22

 

3.4 CBA PROCESS The Cold Bed Adsorption Process is based on the same fundamentals than the Sulfreen Unit, e.g. performing performi ng additional Claus Reaction at low temperatur temperature, e, with subsequent adsorption on the catalyst and a required thermal regeneration step. The difference with the Sulfreen process lies in the fact that the hot gas source for regeneration is taken from the outlet of a Claus reactor. Several configurations are available depending on the number of Claus converters. The temperature of the regeneration gas is around 335 to 350°C. Typically, the feed to the CBA reactor in adsorption is at a temperature of 225°C. Several the Processrange flowscheme are available, of Claus The and most CBA reactors.variations These arrangement from 1 Claus + 2 CBAdepending reactors toonupthe to number 5 CBA reactors. popular arrangements are 1 Claus + 2 CBA reactors and 2 Claus + 2 CBA reactors.  A schematic schematic flow diagram of an arrangement arrangement with 2 Claus Reactors and 2 CBA reactors is enclosed below.

Acid Gas & SWS effluent

Reaction Furnace and Waste Heat Boiler 

R3 CBA #1

Steam

Air from Blower 

R4 CBA #2

Sulphur 

Condenser #1

R1 Claus #1

R2 Claus #2

Condenser #3

Closed Sulphur  Opened

Condenser #4 Sulphur 

CBA Process, R3 in regeneration

Condenser #2

To Incinerator 

Sulphur 

Sulphur Recoveries of 99.3% and 99.4% are expected respectively with a 1+2 and 2+2 arrangements.  About 30 CBA Units are in operation or at design stage with capacities ranging from 2 to 2000 MTPD (expressed as Claus Unit feed sulphur). The CBA considered a true add-on TGCU technique. speaking, at CBA should be technology consideredcannot at the be early stages ofasthe Claus + TGCU design when Generally sulphur recoveries or above 99% are contemplated. Adding CBA reactors downstream of an existing Claus Unit, although feasible, is rather delicate as it implies a significant amount of piping work.

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes  

Page 10 of 22

 

3.5 SUPERCLAUS PROCESS Two main principles are applied in the SuperClaus process: -

Operating the Claus plant with excess H2S to minimize the SO 2 content in the Claus tail gas. This feature simplifies and makes more flexible the air ratio control. Selective oxidation of the remaining H2S in the Claus tail gas by means of specific catalyst which efficiently convert the remaining H2S in the presence of water vapour and excess oxygen to elemental sulphur only. H2S + ½ O2

→

 S + H2O

This reaction takes place in a specific converter (oxidation reactor), downstream of a two or three reactors traditional Claus unit. COS and CS 2 produced in the Claus section are not converted. Recoveries up to 99% (typically 98.5 to 98.7%) are achievable downstream a two reactors Claus Unit. Getting over 99% would require a third Claus reactor and/or and hydrogenation/hydrolysis step. For instance the so-called SuperClaus 99.5 process includes such and hydrogenation/hydrolysis step. Jacobs Comprimo has announced recently the development of a new catalyst to be used upfront of the SuperClaus reactor. This new arrangement has been named EuroClaus.  An overall overall flows flowscheme cheme of a SuperClaus SuperClaus arrangement arrangement is is given by the following following diagram: diagram:

Steam

#1 Reheater  Reaction Furnace and Waste Heat Boiler 

Acid Gas & SWS effluent

#1 Reactor 

#1 Condenser 

FR C

#2 Condenser  Trim Air 

Steam

Control 

Sulphur 

Sulphur 

 Air Bl ower  #2 Reheater  #2 Reactor 

Steam

to Trim Air  AI C Control 

#3 Reheater  Air 

H2S : 0.8 to 1.5 % vol.

O2 : 0.5 to 2% vol.

AI C

#3 Condenser 

SuperClaus Reactor  Sulphur 

Tail Gas to Incinerator 

#4 Condenser 

Sulphur 

SuperClaus 99 Process

SUPERCLAUS technology was initially developed by Stork Engineers & Contractors B.V. (now part of the Jacobs group), and introduced to the industry in 1988. Revamping of an existing Claus Plant to a SuperClaus configuration is relatively straight-forward. A three stage Claus Unit can easily be revamped simply by changing the 3rd stage catalyst (plus some piping and minor equipment modifications). The catalyst used is an Alumina based catalyst coated with iron oxide and chromium oxide oxide layers. It ensures 80 to 90% H2S to be oxidized to sulphur. Other sulphur species (COS, CS 2, SO2) will pass through the catalyst as lost recovery, hence the need to operate the upfront Claus Unit with a high H2S to SO2 ratio at the outlet of the second Claus converter. The catalyst is not sensitive to water. More than 80 plants have been licensed worldwide, with capacities ranging from 7 to 700 MTPD (expressed as Claus Unit feed sulphur).

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes  

Page 11 of 22

 

3.6 CLAUSPOL PROCESS The Clauspol process consists mainly in an absorber/reactor fitted with several layers of packing. The Claus Unit Tail Gas from the last condenser is fed to the bottom part of the Clauspol reactor where it is counter-currently washed with the solvent. The Tail Gas from the reactor is then sent to the downstream incinerator. The reactor operates at about 120°C. This temperature, together with a relatively long residence time, permits a certain degree of COS and CS 2 hydrolysis. Tail Gas to Incinerator 

Clauspol Reactor 

HW

Claus Tail Gas

Sulphur  Drum Catalyst Make-Up

Solvent to Recovery

Liquid Sulphur 

CLAUSPOL II Process

The sulphur produced, being being only slightly miscible with the solvent, is collected at the bottom of the absorber/reactor as a separate liquid phase, given its higher density.  As the reaction is exothermic, the heat of reaction is dissipated by the solvent recirculation loop which is continuously circulated and cooled by a water cooler. Several versions of the Clauspol process have been developed since its initial introduction over 25 years ago. The initial version was named Clauspol 1500. Its Sulphur recoveries were in the range of 98.5% to 99.3%. More than 30 Units with capacities up to 600 MTPD (expressed as feed to the upfront Claus Unit) have been implemented. The Clauspol II version was commercially introduced in 1993 and differs in the method used for solvent temperature control. The initial Clauspol 1500 temperature control was performed by water injection, whereas in the Clauspol II an indirect cooling through a heat exchanger is used. It allows a 350 recovery ofand about 99.6% offrom sulphur. Units sulphur. have been licensed with global capacities ranging from 25 to MTPD recoveries 99 to 499.8%

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes  

Page 12 of 22

 

3.6.1 Clauspol 99.9+

 A more recent development of the Clauspol technology is the Clauspol 99.9+. The enhancement of this version lies in a desaturation loop. Under normal operating conditions, the elemental sulphur in solution (e.g. dissolved) in the solvent is about 2% wt. The amount of sulphur vapour left in the tail gas to the incinerator is therefore in equilibrium with this amount of dissolved sulphur. This represents approximately 300 ppmv sulphur equivalent in the tail gas. In order to further reduce this amount the overhead tail gas has to be contacted with a stream with less dissolved sulphur. Hence the idea of a desaturation loop as depicted on the following flowscheme. This loop oop can be added to an existing Clauspol II Unit. Tail Gas to Incinerator 

Steam

Clauspol Reactor 

HW

Claus Tail Gas

CW Steam

Sulphur  Drum Catalyst Make-Up

Solvent to Recovery

Liquid Sulphur 

CLAUSPOL 99.9+ Process

 

 An overall recovery of 99.9% is claim claimed, ed, provide provided d that the hydrolysis hydrolysis of COS and CS 2 have reached a significant level in the upfront Claus Unit.

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes  

Page 13 of 22

 

3.7 RAR PROCESS This type of process is by far the most widely applied TGCU type of process which can achieve overall Sulphur recovery above 99.9%. The concept underlying H2S scrubbing processes are: § § §

Hydrogenation and hydrolysis of all sulphur compounds to H2S  Absorption of H2S by an Amine solution (generic amine or specialty amine) Regeneration of the Amine solution and recycle of the H2S to the upfront Claus Reaction Furnace.

 A general process process arrangement arrangement is given by the following following scheme: scheme: H2S recycle to Claus Unit Tail Gas to Incinerator  Electric Heater (1)

CW

 Absor ber  Regenerator 

Hydrogen Make-Up (1)

Quench Tower 

Hydrogenation Reactor 

CW

Steam

CW

Claus Tail Gas

Sour  Water 

Typical H2S Scrubbing Process

(1) Other arrangement possible (e .g. RGG)

Hydrogenation reactions are as follows: § §

SO2 + 3 H2 ßà 2 H2O + H2S Sn + n H2  ßà n H2S

Hydrolysis reactions are as follows: §

COS + H2O ßà CO2 + H2S CS2 + 2 H2O ßà CO2 + 2 H2S

§

CO + H2O ßà CO2 + H2 

§

Before the tail gas from the Claus section can be hydrogenated/hydrolyzed, it has to be preheated to above 280°C, in order to activate the hydrolysis/hydrogenation (usually a CoMo type catalyst). Preheating can be performed in different ways, either by direct combustion or by indirect heating by means of high pressure steam or hot oil. Typically, all the CS 2 and about 90% of the COS are converted. Direct combustion is required when no external reducing gas source (Hydrogen and/or CO) is available to complement the hydrogen and CO present in the Claus tail gas. Before the reduced gas enters the downstream Amine Unit, it has to be cooled down to the maximum extent to enhance the absorption by the solvent. This is typically performs in two steps. First the gas is cooled in a Waste Heat Boiler, generating steam at low pressure, and then it is contacted with cold water in a quench tower. A gas/gas exchanger may also be used.

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes  

Page 14 of 22

 

Besides reducing the gas temperature, the purpose of the quench tower is to provide a buffer protection of the downstream Amine unit in case of upsets of the Claus section. Amine solvents are very sensitive to SO2 which produces Heat Stable Salts in the Amine Solution, and therefore a good control of the pH of the circulating water will give an indication of SO2 breakthrough before it reaches the Amine absorber. H2S is absorbed by a countercurrent flow of lean amine as the tail gas flows up in the low pressure absorber. The overhead stream containing typically 10 to 250 ppmv H2S, depending on the solvent used, is sent to the thermal incinerator before being discharged to atmosphere. The rich amine from the bottom of the absorber is pumped to the regenerator where H2S is stripped by means of steam reboilers. The overhead stream is recycled back to the reaction furnace of the Claus section. The operating pressure of the regenerator is set so as to allow a direct recycle. In certain instances where the operating pressure at the outlet of the Claus section is not sufficient to feed the scrubbing section, a dedicated blower has to be installed. It is usually located downstream of the quench tower. There have been numerous variations of this type of process, initially developed by Shell (SCOT process) using various types of Amines and of process configurations in order to: § § § §

Maximize the H2S absorption, Minimize the CO2 coabsorption, Reduce capital cost, Reduce operating costs.

Several Licensor currently propose variations on the H2S scrubbing process, using solvents available on the market place, or in some instances proprietary solvents. Overall sulphur recovery ranges from 99.9 to 99.99% depending mainly on solvent selection. The following development will list the technologies available, and outline potential specific arrangements. It is restricted mainly to Refinery applications. RAR is a generic name for various developments in Tail Gas Clean Up and Acid Gas Enrichment from Technip KTI Spa. For instance, the Multipurpose RAR combines, for lean gases (e.g. with low H2S content) the Acid Gas Enrichment and the Tail Gas H2S scrubbing in the same absorber. This arrangement achieves a dramatic capital cost reduction compared to two separate Acid Gas Enrichment and Tail Gas Clean Up units.

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes  

Page 15 of 22

 

3.8 LO-CAT PROCESS The LO-CAT Process is currently offered for license by US-Filter, a subsidiary of Vivendi. The process was initially developed at the beginning of the seventies by ARI Technologies, which investigate a range of chelating agent which would insure iron solubility under a wide range of pH. The ARI LO-CAT process has been licensed more than 120 times, mainly for small capacities and for applications such as Sour Natural Gas treatment (small fields), bio-gas applications, amine treaters acid gas treatment, … A few references only exists for TGCU, although the process is fully applicable. Several arrangements of the LO-CAT II process have been developed depending on the nature of the gas to be treated. For gases such as natural gas, or CO 2  gas containing H2S (for beverage purpose), a scheme with a separated absorber is usually applied. The absorber is a column fitted with specific internals, and a separate oxidizer is provided. This arrangement prevents the treated gas to be contaminated with air. For gases for which air ingress/contamination is not of concern, the so-called “Autocirculation” arrangement is used. This arrangement would be the preferred one for a TGCU Unit. Vent

Claus Tail Gas  Auto circ ulat ion Vessel

Ai r I n t a k e Filter 

 Air Blower 

Wash Water 

MELTER System Vent

Belt Filter 

Sulphur Cake

Wash Water 

Steam

Filtrate Return Pump

US-Filter LO-CAT II Process

Slurry  Tank 

Sulphur Separator 

Sulphur Slurry Pump

Liquid Sulphur 

 Absorption and regeneration are performed  Absorption performed in a single vessel divi divided ded in two sections: the Centerwell Centerwell and the outer space where aeration with air is performed. The purpose of the Centerwell is to separate the sulphite ions ion s from a air ir in order orde r to minimize minimiz e by -product formation (e.g. thiosulfate). The difference in aeration (and therefore of density) between the Centerwell and the outer space give sufficient driving force for solution circulation between the absorption and the regeneration zones without the need of a specific pump. The last type of processing scheme is called the “aerobic unit” and is used to treat air contaminated with H2S. All reactions take place in the same vessel, at the expense of increased by-product formation, but with the advantage of a reduced capital cost.  As for all the CIP processes, the sulfur produced is under the form of a sulfur cake, which after washing in a belt filter (other types of filters may be used, depending of factors such as unit capacity) contains more than 35% wt of water and a few percent of solution. Should higher sulphur purity be required, then a sulphur melter section is to be installed. LO-CAT process can produce a Tail Gas with as low as 10 ppmv H2S. COS, CS 2  and SO2 being practically inert for the solution, the overall recovery downstream of a Claus Unit is limited to about 98.5 to 99%. Should a higher recovery be required, then an hydrogenation/hydrolysis step is required upstream of the LO-CAT section.

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes  

Page 16 of 22

 

4. COMPARISON METHODOLOGY 4.1 TECHNICAL COMPARISON In order to present a comparis comparison on as accurate as possible of existing Tail Gas Clean-Up technologies, the different processes were compared on the same basis: this implied a common definition for the inlet Claus Tail Gas, climatic conditions, available utilities.  A common basis of design was issued to each licensor of the selected processes and a evaluation package obtained. Subsequently, the performance of each process (percentage of sulphur removal, utility consumption…) was analysed and the different technologies compared.

4.2 CAPITAL C OSTS Each licensor was requested to provide its own estimate of capital cost. To ensure the consistency of the costs prepared by licensors, Technip carried out an independent estimate of the capital cost for each process based on its normal estimating procedure using project data for similar units. The estimate is given with an accuracy of + 30%, but more importantly, the methodology used to develop the estimate of each TGCU process and the parent Claus Unit is the same in all cases which ensures internal consistency. The capital cost is the total installed cost including engineering, procurement and construction in France.  Are excluded from the above estimate the impact on the refinery environment and existing installations, such as the impact on any existing Sour Water Stripper, steam systems, firewater and buildings, control room and other site facilities.

4.3

OPERATING COSTS

The costs related to operator supervision, utility consumption and production and the revenue from sale of the additional sulphur produced were based on European averages. Catalyst, solvent and chemical costs used for the economic evaluations were provided by the process licensors. The climatic conditions are those of Western Europe.

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes  

Page 17 of 22

 

5. RESULTS 5.1 PROCESS PERFORMANCE 5.1.1 5.1 .1 Expected Overall Sulphur Recovery and Additional Additional Recovered Recovered Sulphur The following table indicates the expected overall sulphur recovery yield, the resulting additional recovered sulphur and the dry basis sulphur emission (in the form of SO2) after incineration.

Process Claus Sulfreen HydroSulfreen (1) DoxoSulfreen (2) CBA Superclaus (3) Clauspol II RAR LO-CAT II (4)

Expected Sulphur

Expected Additional

Expected Sulphur

-

Recovery Yield (%) 96.01

Sulphur Recovered t/d - 

Emission (Dry3Basis) mg / Nm   13652 

Lurgi Lurgi Lurgi BPVI BPVI Jacobs IFP KT KTII US Filter

99.42 99.67 99.88 99.50 98.66 99.60 99.94 99.99

3.56 3.82 4.04 3.65 2.77 3.75 4.10 4.16

2010 1066 414 1726 4631 1382 242 18

Licensor

(1) Sulfreen reactors reactors and hydrolysis hydrolysis section (2) Sulfreen reactors, hydrolysis section and DoxoSulfre DoxoSulfreen en reactors reactors (3) Technip Techni p estimate estima te (4) As LO-CAT II tail gas cannot be incinerated, sulphur is in the form of H2S.

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes  

Page 18 of 22

 

5.1.2 Expected Streams Flows and Compositions Compositions The tables below give the expected TGCU exhaust compositions for sulphur compounds as well as compositions of the streams emitted to atmosphere after being incinerated at 800°C with an O 2 excess of 3% vol. (on a wet basis). The two first columns deal with the reference Claus unit alone, depicting respectively the tail gas at the outlet of the third condenser and the stream from stack to atmosphere after being incinerated at 800°C with an O2 excess of 3% vol. on a wet basis.

Claus Tail Gas Vent Pressure Temperature Flowrate   H2S CS2  COS SO2  S1 eq.

( bar g) (°C) (kmole/ (km ole/h) h) (kmole/h) (kmole/h) (kmole/h) (kmole/h) (kmole/h)

Sulfr Sulfreen een Tail Gas Vent

HydroSulfreen Tail Gas Vent

DoxoSulfre DoxoSulfreen en Tail Gas Vent

0.45 0 .0 0 0.05 0 .0 0 0.05 0.00 0.05 0.00 128.2 800.0 140.1 800.0 140.1 800.0 12 5 800.0 434.037 434.037 759 759.701 .701 432.415 432.415 752.434 752.434 429. 429.950 950 747. 747.966 966 429. 429.052 052 750 750.759 .759 3.261 0.051 0.239 1.630 0.162

0.000 0.000 0.000 5.400 0.000

0.265 0.050 0.240 0.133 0.043

0.000 0.000 0.000 0.781 0.000

0.250 0.005 0.024 0.125 0.043

0.000 0.000 0.000 0.452 0.000

0.071 0.005 0.024 0.013 0.043

0.000 0.000 0.000 0.161 0.000

  CBA Tail Gas Vent Pressure (bar g) Temperature ( ° C) Flowrate (kmole/h) (kmole/ h)

0.03 1 26 26 438.090 438.090

0 ..0 00 800.0 76 764.91 4.918 8

(kmole/h) (kmole/h) (kmole/h) (kmole/h) (kmole/h)

0.370 0.020 0.000 0.19 0.190 0 0.080

0.000 0.000 0.000 0 0.6 .680 80 0.000

H2S CS2  COS SO2  S1 eq.

SuperClau SuperClaus s Tail Gas Vent

Clauspol Tail Gas Vent

0 ..0 05 0.00 132.0 800.0 457.70 457.704 4 756. 756.371 371

0 ..0 05 120.0 432. 432.395 395

0 ..0 00 800.0 7 758.9 58.930 30

0.000 0.000 0.000 1.8 1.812 0.000

0.121 0.043 0.144 0.06 0.061 1 0.130

0.000 0.000 0.000 0 0.5 .542 42 0.000

0.143 0.051 0.239 1.30 1.304 4 0.024

RAR Tail Gas Vent

LO-CAT Vent

0 ..0 05 0.00 41 1..0 800.0 31 310.03 0.039 9 555. 555.311 311

0 .0 .05 5 2. 2.1 4 449.7 49.785 85

0.000 0.000 0.000 0.084 0.000

0.005 0.000 0.000 0.000 0.000

0.084 0.000 0.000 0.000 0.000

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes  

Page 19 of 22

 

5.2 CAPITAL AND OPERATING COST ESTIMATES 5.2.1 Capital Costs Estimates Usual practice is to relate the capital cost of the TGCU to the one of the upfront Claus unit. The following table gives estimates of such a ratio, for a 100 MTPD Claus unit (including catalyst) in a Refinery environment. Without Licence, Catalysts Cataly sts and Chemicals Process (%)   (%)  Sulfreen 29.2 HydroSulfreen (1) 44.7 DoxoSulfreen (2) 67.0 CBA 35.4 Superclaus (3) 12.3 Clauspol II 33.7 RAR 67.2 LO-CAT II 46.8

Licence, Catalyst and Chemicals included (%)   30.9 47.6 76.0 36.1 15.3 37.3 67.5 49.0

(1) Sulfreen Sulfreen reactors and hydrolysis section (2) Sulfreen reactors, reactors, hydrolysis hydrolysis section and DoxoSulfreen reactors (3) Technip Techni p estimate estima te The reference to the upstream sulphur unit is indicative and corresponds to the way this kind of comparison is usually presented in the literature. This comparison should be taken with care when comparing with other studies, as the capital cost of a sulphur unit may vary for a number of reasons which are developed below: §

Capacity and degree of modularization Small size plants (capacities below approximately 60 to 80 MTPD on rich gas) could be preassembled in one of several modules, therefore reducing the erection cost which otherwise can represent between 30 and 40% of the total erected cost of the unit. In addition, for capacities up to 200 MTPD, several pieces of equipment in the sulphur recovery unit could be grouped in the same vessel or shell (Claus reactors #1 and #2, Claus condensers #2 and #3), therefore minimizing the equipment cost. Sulphur recovery unit capital cost estimate is based on separate equipment with no modularization. This estimate was performed in a manner consistent with the downstream TGCU Units. This estimate is probably on the high side of capital cost, therefore reducing the ratio TGCU / SRU.

§

 Acid Gas quality qu ality Depending gas H2S content, the impact on the sulphur recovery unit and the TGCU configurationon canthe be acid different.

The following points shall also be highlighted: §

The same methodology was followed for each unit, which provides internal consistency.

§

The estimated accuracy for capital cost estimate is + 30%.

§

CBA capital cost is the most difficult cost to estimate, as, depending of the actual configuration of the Claus unit, the extent of piping works modification may vary from Plant to Plant. The current capital cost assumes that modifications to piping and equipment are minor.

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes  

Page 20 of 22

 

5.2.2 Operating Cost Estimates The estimate of operating costs, including sulphur produced, utilities and chemicals as well as additional manpower expense are given below.

Process Sulfreen HydroSulfreen DoxoSulfreen CBA Superclaus Clauspol II RAR LO-CAT II

5.3

OPERATING COST Operation Recovered TOTAL Cost Sulphur k$ / y k$ / y k$ / y 103 -24 79 154 -26 128 363 390 -27 59 -25 34 109 128 -19 124 -25 99 161 189 -28 331 -28 303

ESTIMATED RECOVERY COST OF SO2 

The estimated ranges for recovery cost of SO 2 for each process are given in the following table.

Proc ess Process SuperClaus Sulfreen CBA Clauspol II HydroSulfreen DoxoSulfreen RAR LO-CAT II

Expected Sulphur Recovery Yield (%) 98.66 99.42 99.50 99.60 99.67 99.88 99.94 99.99

Expected Additional Sulphur Recovered (t / d) 2.77 3.56 3.65 3.75 3.82 4.04 4.10 4.16

Recovery Cost Ranges of SO2  ($ / t) 137 / 202 154 / 256 150 / 266 176 / 292 224 / 370 404 / 624 284 / 477 271 / 409

Calculations have been performed based on the following hypothesis: -

The TGCU cost includes license fee, catalyst and first fill of chemicals. These investments are financed by a fully reimbursable 10-year loan with a 6% interest rate.

-

The catalyst lifetime is 3 years (part of the operating costs). The related investment every 3 years is financed by a fully reimbursable 3-year loan with a 6% interest rate.

-

The solvent and chemical make-up make-up (part of the operating costs) were calculated calculated on a yearly basis as a cash expense.

-

The consumption and production of utilities as well as the manpower costs were considered constant over time.

-

The price of sulphur was also considered constant over time.

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes  

Page 21 of 22

 

6. CONCLUSIONS The conclusion is in the form of a graphical comparison showing for each TGCU studied q

Overall sulphur recovery for the Claus plant plus TGCU (%)

q

The range of costs for recovery of non-rejected SO 2 for each TGCU studied reflecting the +/-30% accuracy of the capital cost estimate.

Recovery SO2 ($Cost / t) of 700

TAIL GAS CLEAN-UP

 Yield  Yie ld (%) (%)

100.00

600 500

99.50

400 99.00

300

200 98.50

100

Estimated Re Recovery Co Cost of of SO SO2

Sulphur R Re ecovery Yi Yield

  GPA Europe Technical Meeting, Paris, 21st February, 2003 Evaluation of TGCU Processes  

Page 22 of 22